It's abandoned ad lib! (6)
I believe the answer is:
script
'abandoned ad lib' is the definition.
The definition suggests a singular noun which matches the answer.
'it's' is the wordplay.
I cannot quite understand how this works, but
'it' could be 't' (abbreviation. e.g. in 'tis) and 't' is present in the answer.
The remaining letters 'scrip' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for script that I've seen before include "Printed text" , "Piece of handwriting" , "Written copy of words to be spoken" , "Words of film etc" , "Calligraphy" .)